1. Field of Invention
This invention is related to driver circuits and in particular audio output drivers for piezoelectric speakers.
2. Description of Related Art
A large part of the power consumption of an audio device is the output stages or power of the power amplifier section. It is particularly critical for portable devices powered by batteries to conserve power to reduce battery drain. Most audio devices use traditional electro-magnetic speakers, which present an electrical resistance and consume power as a result of the resistance, wherein only a small portion of the output energy is converted to an audio signal. The inefficiency of converting electrical energy to sound using electro-magnetic speakers is a reality with all audio devices regardless of the efficiency of the power amplifier section.
Another type of audio speaker uses piezoelectric technology that in general provides a capacitive load. Piezoelectric sound devices are not new; however, applications of piezoelectric sound producing devices in general have been limited to simple sounding devices, for instance alarms, tones and simple sounds, but not in general audio signals such as voice.
US 2002/0126001 A1 (Baldwin et al.) is directed to a piezoelectric transducer driving circuit that has a main oscillator stage, a buffer circuit and a voltage doubling circuit. U.S. Pat. No. 7,161,263 B2 (von Styp-Rekowski et al.) is directed to a piezoelectric driving and switching apparatus, which includes a power supply circuit having a supply open configuration and positive and negative configurations for transferring to an inductor. U.S. Pat. No. 7,070,577 B1 (Haller et al.) is directed to an implantable beneficial agent infusion device featuring a unique energy recovery circuit and a deflectable energy storing member. U.S. Pat. No. 6,016,040 (Hoffmann et al.) is directed to a device for driving capacitive actuator, which contains a charge capacitor and a discharge capacitor. In U.S. Pat. No. 5,262,757 (Hansen) an electronic warning device, which includes a power source, is directed to a motion detector, a pleasant tone generator, a loud tone generator and a remote activation switch. In U.S. Pat. No. 5,126,589 (Renger) a piezoelectric drive circuit is directed to use with a capacitive load including two transistors controlled by a drive voltage that is connected to the load by an inductor. U.S. Pat. No. 4,947,074 (Suzuki) is directed to a piezoelectric device drive circuit that comprises a charge circuit and a switch circuit connected between the terminals of the piezoelectric device. In U.S. Pat. No. 4,498,089 (Scardovi) a control circuit is directed to an ink jet printing element, wherein individual drops of ink are expelled from a container by way of the contractions piezoelectric transducer. In U.S. Pat. No. 4,109,174 (Hodgson) a drive circuit is directed to a piezoelectric device, which comprises and inductor wherein energy from the inductor is transferred to the piezoelectric device.
Because of the capacitive nature of piezoelectric sound devices, a speaker for instance, energy is stored as a voltage across the terminals of the device as current flows into the sound device. This energy is lost when the driver circuit reverses the polarity of the output signal of the driver circuit, and the piezoelectric speaker, or sound device, is discharged. If this lost charge could be recovered, then the power consumption of the driver circuit could be reduced with only a small reduction in audio volume since the sound device, or speaker, will vibrate in the same way whether the charge is lost in discharge or recovered in some manner.